The present invention relates to an apparatus and method for balancing a Coriolis-type mass flowmeter, particularly one which comprises a single straight flowtube, over all conditions and frequencies to which the flowtube may be subjected during operation of the flowmeter
Prior art Coriolis mass flowmeters typically include a flowtube through which a fluid to be measured is directed, electromagnetic force generating means for vibrating the flowtube in one of its modes of vibration, such at its 1st natural bending mode of vibration, and one or more transducers for measuring the vibrational deflection, or shape, of the flowtube. The vibrating flowtube causes the Coriolis forces which are generated by the flowing fluid to bear against the wall of the flowtube and thus alter the vibrational shape of the flowtube. The altered vibrational shape is measured by the transducers and provides an indication of the flow rate of the fluid, as is well understood by those skilled in the art.
The flowtube comprises end connections, such as conventional pipe flange connections, through which the flowtube is connected to a supporting structure, such as a fluid flow pipe, and to which other components of the flowmeter may be secured. These supporting structures and other flowmeter components, which together are referred to herein as the support structure for the flowtube, define boundary conditions for the flowtube which may influence the operation of the flowmeter. The vibrations in the flowtube react against the support structure through the end connections, and these reaction forces can excite the support structure and consequently drain energy from the vibrating flowtube and result in erroneous flow rate readings.
Several mechanisms have been employed in prior art Coriolis mass flowmeters to counterbalance these deleterious reaction forces. For example, one prior art flowmeter comprises a fixed counterbalance which is similar in shape to the flowtube and includes weights to simulate the density of the fluid to be measured. Thus, when the flowtube is vibrated against this counterbalance device, the reaction forces generated by the counterbalance device will nullify the reaction forces from the flowtube at the end connections. However, these meters were found to be highly sensitive to ambient vibrations and changes in the boundary conditions of the flowtube. Another prior art flowmeter comprises two identical parallel flowtubes. The fluid to be measured is directed through both flow tubes and the flowtubes are vibrated in opposition to each other. Therefore, the tubes remain in near perfect balance regardless of changes in the fluid parameters or the boundary conditions of the flowmeter. Consequently, any resultant reaction forces that may be present at the end connections of the flow tubes are negligible. However, splitting the flow stream into two paths creates a pressure loss and turbulence and can also result in one flowtube becoming plugged. Other more recent attempts to effectively counterbalance the reaction forces from the flowtube rely on fixed counterbalance designs, which as mentioned above have some sensitivity to changes in the boundary conditions.
The present invention addresses these and other limitations in the prior art by providing a Coriolis mass flowmeter with a counterbalance system for balancing the vibrating flowtube to thereby nullify the reaction forces that the flowtube would otherwise induce in the support structure. The condition of balance between the vibrating flowtube and the counterbalance requires that the reaction forces generated by these structures be equal and opposite, thereby canceling each other at the point where the structures are coupled. This will result in the vibration of the system (that is, the vibration of the flowtube and the counterbalance) being contained and therefore isolated from any changes in the support structure or boundary conditions of the flowtube. To achieve balance between the flowtube and the counterbalance during all phases of operation of is the flowmeter, the counterbalance system must be able to change its frequency response to match that of the flowtube, which itself can change drastically in response to changing fluid parameters such as density, pressure and viscosity. To accomplish this, the present invention essentially provides a variable mass/spring/damper counterbalance system that is controlled by a current to alter the frequency response of the counterbalance as necessary to match the frequency response of the vibrating flowtube.
According to the present invention, therefore, a Coriolis-type mass flowmeter for measuring the flow rate of a fluid is provided which comprises a flowtube having first and second ends, a counterbalance which is vibrationally coupled to the flow tube proximate the first and second ends, means for vibrating the flowtube and the counterbalance in opposition to one another, at least one inertial mass, and means for selectively coupling the inertial mass to the counterbalance. In this manner, the frequency response of the counterbalance to the vibrating means can be changed to approximate the frequency response of the flowtube to the vibrating means by coupling the inertial mass to the counterbalance.
In the preferred embodiment of the invention, the counterbalance comprises an elongated beam which is designed to have a mass and stiffness distribution along its length similar to that of the flowtube for a given condition, such as when the flowtube is filled with water. The counterbalance is therefore designed to vibrate in a similar fashion, at the same frequency, and in opposition to the flowtube and thereby create equal and opposite reaction forces at the first and second ends of the flowtube, which will result in a balanced condition. In the event the flowmeter may need to measure fluids other than water, however, the frequency response of the flowtube may be altered by such fluids to the extent that it no longer matches the frequency response of the counterbalance, thus resulting in an unbalanced condition.
The present invention addresses this problem by the inclusion of the inertial masses. In the preferred embodiment of the invention, the inertial masses are disposed along the length of the counterbalance and can be coupled to or uncoupled from the counterbalance by application of a control current or signal. These inertial masses are suspended by a low frequency mounting system so that, when they are uncoupled, they contribute substantially no additional mass, stiffness, or damping to the counterbalance and therefore do not substantially alter its frequency response. If the flowmeter becomes unbalanced, one or more of the inertial masses can be either wholly or partially coupled to the counterbalance to add substantial mass and/or damping to the counterbalance and thereby alter its frequency response as necessary to match that of the flowtube. By controlling the coupling of the one or more inertial masses to the counterbalance, the requisite condition of balance can thus be restored xe2x80x9con the flyxe2x80x9d over any desired range of fluid conditions.
In the preferred embodiment of the invention, the inertial mass comprises a cylindrical steel support member which is fixed to the counterbalance and an iron or steel cylinder or xe2x80x9cbobbinxe2x80x9d which is mounted within the support member by one or more flexural members. The flexural members permit the bobbin to move relatively freely in the direction of vibration of the counterbalance but restrict the movement of the bobbin in the transverse direction.
The preferred means for coupling the inertial mass to the counterbalance comprises an electrical coil which is preferably wrapped around the bobbin and a Magneto-Rheological fluid (xe2x80x9cMRFxe2x80x9d) which is disposed in a gap between the bobbin and the support member. When it is desired that the mass be uncoupled from the counterbalance, no current is applied to the coil, and the bobbin is thus free to stay essentially motionless while the counterbalance vibrates. When it is desired that the bobbin be coupled to the counterbalance, a current is applied through the coil. Upon the application of the current, a magnetic field is produced in the gap which causes the MRF to stiffen, effectively coupling the bobbin to the counterbalance. In this coupled condition, the bobbin will vibrate substantially with the counterbalance. This additional mass then alters the frequency response of the counterbalance. By coupling or uncoupling one or more masses to the counterbalance, the desired balanced condition can be achieved regardless of the particular conditions of the fluid in the flowtube.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings.